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irina irina
wrote...
Posts: 919
11 years ago
The mole fraction of potassium nitrate in an aqueous solution is 0.0194.  The solution density is 1.0627g/mL.  calculate the molarity of the solution.

Ok i am very very stupid and I have to know how to solve these types of problems so can you explain this in detail.  And can you give me some advice explaining how you approached this problem and stuff.  Because when I tried it was clueless, so I need to learn some techniques.  Please help me, I'm in dire need of help.
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wrote...
11 years ago
This one's pretty tricky... (You're NOT stupid.)

Let's start with some relevant definitions:
A solution comprises a solute, the thing being dissolved, and a solvent, the thing doing the dissolving.

mole fraction = (moles of solute) / (moles solution) [1]

molarity = (moles of solute) / (liters of solution) [2]

For this conversion, let's use 1 mole of solution to make the calculation easier and the concepts a bit more clear (though it really doesn't matter how much we use).
From [1]:
0.0194 = (moles of solute) / 1
moles of solute = 0.0194

moles of solution = (moles of solute) + (moles of solvent)
1 = 0.0194 + (moles of solvent)
moles of solvent = 0.9806

The only thing given in the problem to help us with converting to "liters of solution" is the density of the solution (in g/ml). We'll first have to convert the moles of solute and moles of solvent to grams in order to use the density for conversion to liters:
g solute = (moles of solute)(molar mass of solute KNO3)

molar mass of KNO3 = 39.098 + 14.007 + 3*15.999 g/mol = 101.102 g/mol

g solute = (0.0194 mol)(101.102 g/mol) = 1.96 g
---

g solvent = (moles of solvent)(molar mass of solvent H2O)

molar mass of H2O = 2*1.0079 + 15.999 g/mol = 18.0148 g/mol

g solvent = (0.9806 mol)(18.0148 g/mol) = 17.67 g
---

g solution = (g of solute) + (g of solvent)
g solution = 1.96 + 17.67 g = 19.63 g

Now we can convert g solution into L of solution:
(19.63 g)(ml / 1.0627 g)(L / 1000 ml) = 0.01847 L

From [2]:
molarity = (0.0194 mol) / (0.01847 L) = 1.05 M
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